(Photo: Thomas Allen)

In February, the media started tittering over erotic romance trilogy Fifty Shades of Grey, by British author E. L. James. By that point, the BDSM-tinged novels, about virginal college student Anastasia Steele and her punishing paramour, billionaire Christian Grey, had sold about 100,000 e-books and print-on-demand paperbacks, owing mostly to Amazon reviews and word of mouth. A month later, amplified by scandalized coverage from the New York Times and the Today show, the buzz grew, sales tripled, and a bidding war broke out for the movie rights. But while mainstream attention for a work of erotica is unusual (in this case, it probably helped that the book was a hit in Manhattan instead of just the South and Midwest, where books like it are more popular), Fifty Shades’ pre-titter sales were no major anomaly: “Once in a while, a random book will just pop,” says Kimberly Whalen, a literary agent with Trident Media Group. “It’s incredibly successful, but there are a lot of other erotic books selling well.”

If that surprises you, consider that romance, the large umbrella under which erotica is broadly classified, is a nearly $1.4 billion, recession-proof industry and the top-selling genre of fiction in the U.S., beating mystery and sci-fi combined. And if the bondage in Shades isn’t your thing, there’s probably a subgenre that is: More than just the classic historical bodice ripper, the spectrum of romantic fiction now includes everything from sex-free books starring Amish lovers to racier ones involving werewolves and vampires to the burgeoning sub-sub-category of erotic paranormal cowboy-ménage romance. For the uninitiated, here’s a primer on the vast, ever-expanding universe of smut.